Not my current job but my last one, I was a developer for a major web app used by every employee in the company. I was working on a toast notification system so I had an icon of toast. You clicked it, and it just showed a notification that said “Toast.” This was on our dev system that people had access to if they wanted to try new upcoming features.
People loved this for some reason, but naturally once I ironed out the bugs the icon went away. But if you entered the Konami code on the main page it faded in a giant version of the toast icon with a dramatic “dun-dun-DUNNNNNNN”. This Easter egg persisted into production.
Is there anywhere else? I swear they look for carpet when they need to puke. If you have a single carpeted room in a house full of laminate, they’re running to that damn carpeted room to blow their gut’s airlock.
Where does the sound come from? Your headphones, speakers, etc. Does it ever happen when your machine is off? You mentioned you only have wired audio peripherals - perhaps someone is playing a prank on you and has connected some kind of device inline.
Gamify the process. Get into Pokemon Go, Ingress, or some other game that requires travel. During each installation, leave behind a little Easter egg. Not for the customer, but for the next tech who has to service the meter. Change it up.
That seems like a waste of perfectly good ingredients for some fake internet points.